docs: def _get_axis_bounds(self, dim, bunch):
          """Return the min/max of an axis."""
          if dim in self.attributes:
              # Attribute: specified lim, or compute the min/max.
              vmin, vmax = bunch['lim']
              assert vmin is not None
              assert vmax is not None
              return vmin, vmax
          # PC dimensions: use the common scaling.
          return (-1. / self.scaling, +1. / self.scaling)